The present invention relates to a method and a device for selecting the appropriate contrast grade photographic paper for printing a back and white transparency.
The contrast of the printed image from a negative can be controlled by the selection of photographic paper used in printing a negative or transparency. Black and white printing paper comes in a range of contrast grades. The normal contrast grade for paper generally is number two. Manufacturers generally produce contrast grades numbering from zero (very low contrast) to number five or six (very high contrast) in half steps.
An alternative to using graded-contrast paper is to print negatives with variable-contrast (also known as selective-contrast) paper. Only one kind of paper is provided and the photographer controls the contrast of the printed image by inserting variable-contrast filters between the light source of the enlarger and the variable-contrast paper. These filters are different colors and change the contrast of the printed image. These filters are generally available in the same contrast grades as for graded photographic paper.
When a negative is low in contrast the photographer selects a higher contrast grade of paper or, if using variable-contrast paper, a higher contrast grade filter. When a negative is high in contrast, the photographer selects a lower number contrast grade paper or variable-contrast filter. If a negative has a section that requires a different contrast grade than another, a photographer can expose the variable-contrast photographic paper with the negative but use different variable-contrast filters for different parts of the negative. This gives the photographer control of local contrast as well as contrast of the whole negative.
Contrast is defined as the density variation between light and dark objects in a negative or print. For example, if the same scene is photographed with two different films or printed with two different contrast grades of paper, one image may show a greater difference in density between light and dark objects in the scene. If so, the developed film or photographic paper with the larger density variation for the same scene produces more image contrast. The contrast of a film or photographic paper are affected by a number of factors including the actual contrast of the subject, the length of exposure, the type of film or paper, the temperature of the developer, the amount of agitation of the developer, the type of developer and the length of development time, among others.
Most printing papers for black and white negatives have a base material coated with a layer of light sensitive emulsion. Variable contrast papers, however, have two such layers or act as if they have two such layers--an emulsion which is sensitive to blue light and another which is sensitive to green light. The blue sensitive layer produces a high-contrast image; the green-sensitive layer gives a low-contrast image. You can get the desired level of contrast by using a filter in the enlarger head or under the lens. In this way you can control whether you use one emulsion layer or the other, or a certain proportion of each. The variable-contrast filters are either magenta or yellow. The yellow filter absorbs blue light, so that the green sensitive emulsion layer is affected--giving an image of relatively low contrast. The magenta filter absorbs green light, so that the blue-sensitive layer is affected--to give higher image contrast. Intermediate contrast is obtained by using filters of greater or lesser strengths. Because the variable-contrast filters are often of different densities and because the variable-contrast paper may have a different degree of sensitivity to equivalent amounts of the blue or green light, as a filter is changed, the photographer may have to increase or decrease the exposure time.
U.S. Pat. No. 2,226,167 describes a projection print scale. This product is a piece of clear film that is placed on the black and white photographic printing paper while the paper is exposed to the negative for 60 seconds. The projection print scale has ten sections (in the form of a circle when ten segments or "pie slices"). Each section is equivalent to a neutral density filter that reduces the exposure to the photographic paper. When the photographic paper is developed, the photographer can look at the section that appears best and read an appropriate exposure.